Strong Southern Ocean carbon uptake evident in airborne observations

Science ◽  
2021 ◽  
Vol 374 (6572) ◽  
pp. 1275-1280
Author(s):  
Matthew C. Long ◽  
Britton B. Stephens ◽  
Kathryn McKain ◽  
Colm Sweeney ◽  
Ralph F. Keeling ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Carbon Uptake ◽  
Ocean Carbon

scholarly journals Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection

2016 ◽  
Vol 43 (5) ◽  
pp. 2077-2085 ◽  
Author(s):  
C. D. Nevison ◽  
M. Manizza ◽  
R. F. Keeling ◽  
B. B. Stephens ◽  
J. D. Bent ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Carbon Uptake ◽  
Future Projection ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon ◽  
Potential Oxygen

scholarly journals TREE RING RECONSTRUCTION OF MODERN RADIOCARBON DIOXIDE VARIABILITY OVER THE SOUTHERN OCEAN

2021 ◽  
Author(s):  
Rachel Corran
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Tree Ring ◽  
Deep Water ◽  
Latitudinal Gradient ◽  
Carbon Sink ◽  
Carbon Uptake ◽  
Co2 Uptake ◽  
Anthropogenic Carbon ◽  
Ocean Carbon

<p><b>The Southern Ocean is the largest ocean carbon sink region. However, its trend of increasing carbon uptake has shown variability over recent decades. It is important to understand the underlying mechanisms of anthropogenic carbon uptake such that the future response of the Southern Ocean carbon sink under climate forcing can be predicted. </b></p><p>The carbon uptake of the Southern Ocean is characterised by the balance of outgassing of CO2 from carbon-rich deep water and sequestration of anthropogenic carbon into surface waters. Atmospheric radiocarbon dioxide (Del14CO2) in the Southern Hemisphere is sensitive to the release of CO2 from the upwelling of ‘old’ 14C-depleted carbon-rich deep water at high southern latitudes, but is insensitive to CO2 uptake into the ocean. Thus Del14CO2 has the potential to be used as a tracer of the upwelling observed, thereby isolating the outgassing carbon component. </p><p>The Southern Ocean Region has limited atmospheric Del14CO2 measurements, with sparse long-term sampling sites and few shipboard flask measurements. Therefore in this PhD project I exploit annual growth tree rings, which record the Del14C content of atmospheric CO2, to reconstruct Del14CO2 back in time. Within tree ring sample pretreatment for 14C measurement I automate the organic solvent wash method at the Rafter Radiocarbon Laboratory. I present new annual-resolution reconstructions of atmospheric Del14CO2 from tree rings, from coastal sites in New Zealand and Chile, spanning a latitudinal range of 44 S to 55 S, for the period of interest, 1985 – 2015. Data quality analysis using a range of replicate 14C measurements conducted within this project leads to assignment of apx 1.9 ‰ uncertainties for all results, in line with atmospheric measurements. </p><p>In this project I also develop a harmonised dataset of atmospheric Del14CO2 measurements in the Southern Hemisphere for this period from different research groups, including the new tree ring Del14CO2 records alongside existing data. The harmonised atmospheric Del14CO2 dataset has a wide range of applications, but specifically here allows investigation of temporal and spatial variability of atmospheric Del14CO2 over the Southern Ocean over recent decades, thereby also considering the role of upwelling in recent Southern Ocean carbon sink variability. Backward trajectories are produced for the tree ring sites from an atmospheric transport model, to help inform interpretation of results. </p><p>Over recent decades a latitudinal gradient of 3.7 ‰ is observed between 41 S and 53 S in the New Zealand sector, with a smaller gradient of 1.6 ‰ between 48 S and 55S in the Chile sector. This is consistent with other studies, with the spatial variability of atmospheric Del14CO2 attributed to air-sea 14C disequilibrium associated with carbon outgassing from 14C-depleted carbon-rich deep water upwelling at around 60 S, driving a latitudinal gradient of atmospheric Del14CO2 in the Southern Hemisphere, with longitudinal variability also observed. A stronger atmospheric Del14CO2 latitudinal gradient is observed in the 1980s/1990s relative to later 1990s/2000s. Stronger atmospheric Del14CO2 latitudinal gradients observed in 1980s/1990s suggest stronger deep water upwelling thereby greater associated outgassing of 14C-depleted CO2. These Del14CO2-based observations are consistent with modelling studies that predict changes in deep-water upwelling have controlled decadal variability in CO2 uptake, and are consistent with observation-based studies of decadal changes in rate of CO2 uptake of the Southern Ocean. The results presented in this thesis present the first observation-based confirmation that decadal changes in the strength of deep-water upwelling can explain decadal changes in the rate of CO2 uptake. </p>

Modeling the physical drivers of the decadal variability of the Southern Ocean carbon uptake

2021 ◽  
Author(s):  
Lavinia Patara ◽  
Torge Martin ◽  
Ivy Frenger ◽  
Jan Klaus Rieck ◽  
Chia-Te Chien
Keyword(s):  
Southern Ocean ◽  
Decadal Variability ◽  
Open Ocean ◽  
Global Ocean ◽  
Carbon Uptake ◽  
Positive Trend ◽  
The Past ◽  
Rate Of Increase ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon

&lt;p&gt;Observational estimates point to pronounced changes of the Southern Ocean carbon uptake in the past decades, but the mechanisms are still not fully understood. In this study we assess physical drivers of the Southern Ocean carbon uptake variability in a suite of global ocean biogeochemistry models with 0.5&amp;#186;, 0.25&amp;#186; and 0.1&amp;#186; horizontal resolution as well as in a 3-member ensemble performed with an Earth System Model (ESM) sharing the same ocean biogeochemistry model. The ocean models show a positive trend of the Southern Ocean CO&lt;sub&gt;2&lt;/sub&gt; uptake in the past decades, with a weakening of its rate of increase in the 1990s. The 0.1&amp;#186; model exhibits the strongest trend in the Southern Ocean carbon uptake.&amp;#160;&lt;span&gt;Different physical drivers of the carbon up&lt;/span&gt;take variability and of its trends (such as changes in stratification, ventilation, overturning circulation, and SST) are analyzed. A particular focus of this study is to assess the role of open-ocean polynyas in driving Southern Ocean carbon uptake. Open-ocean polynyas in the Southern Ocean have pronounced climate fingerprints, such as reduced sea-ice coverage, heat loss by the ocean and enhanced bottom water formation, but their role for the Southern Ocean carbon uptake has been as yet little studied. To this end we analyze conjunctly ESM simulations and an ocean-only sensitivity experiment where open-ocean polynyas are artificially created by perturbing the Antarctic freshwater runoff. We find that enhanced CO&lt;sub&gt;2&lt;/sub&gt; outgassing takes place during the polynya opening, because old carbon-rich waters come in contact with the atmosphere. The concomitant increased uptake of anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; partially compensates the CO&lt;sub&gt;2&lt;/sub&gt; outgassing. When the polynya closes, the ocean CO&lt;sub&gt;2&lt;/sub&gt; uptake increases significantly, possibly fueled by abundant nutrients and higher alkalinity brought to the surface during the previous convective phase. Our results suggest that open-ocean polynyas could have a significant impact on the Southern Ocean CO&lt;sub&gt;2&lt;/sub&gt; uptake and could thus modulate its decadal variability.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

scholarly journals Impact of Historical Climate Change on the Southern Ocean Carbon Cycle

2008 ◽  
Vol 21 (22) ◽  
pp. 5820-5834 ◽  
Author(s):  
R. J. Matear ◽  
A. Lenton
Keyword(s):  
Climate Change ◽  
Carbon Cycle ◽  
Southern Ocean ◽  
Carbon Uptake ◽  
Saturation State ◽  
Aragonite Saturation ◽  
Freshwater Fluxes ◽  
Anthropogenic Carbon ◽  
Natural Carbon ◽  
Ocean Carbon

Abstract Climate change over the last several decades is suggested to cause a decrease in the magnitude of the uptake of CO2 by the Southern Ocean (Le Quere et al.). In this study, the atmospheric fields from NCEP R1 for the years 1948–2003 are used to drive an ocean biogeochemical model to probe how changes in the heat and freshwater fluxes and in the winds affect the Southern Ocean’s uptake of carbon. Over this period, the model simulations herein show that the increases in heat and freshwater fluxes drive a net increase in Southern Ocean uptake (south of 40°S) while the increases in wind stresses drive a net decrease in uptake. The total Southern Ocean response is nearly identical with the simulation without climate change because the heat and freshwater flux response is approximately both equal and opposite to the wind stress response. It is also shown that any change in the Southern Ocean anthropogenic carbon uptake is always opposed by a much larger change in the natural carbon air–sea exchange. For the 1948–2003 period, the changes in the natural carbon cycle dominate the Southern Ocean carbon uptake response to climate change. However, it is shown with a simple box model that when atmospheric CO2 levels exceed the partial pressure of carbon dioxide (pCO2) of the upwelled Circumpolar Deep Water (≈450 μatm) the Southern Ocean uptake response will be dominated by the changes in anthropogenic carbon uptake. Therefore, the suggestion that the Southern Ocean carbon uptake is a positive feedback to global warming is only a transient response that will change to a negative feedback in the near future if the present climate trend continues. Associated with the increased outgassing of carbon from the natural carbon cycle was a reduction in the aragonite saturation state of the high-latitude Southern Ocean (south of 60°S). In the simulation with just wind stress changes, the reduction in the high-latitude Southern Ocean aragonite saturation state (≈0.2) was comparable to the magnitude of the decline in the aragonite saturation state over the last 4 decades because of rising atmospheric CO2 levels (≈0.2). The simulation showed that climate change could significantly impact aragonite saturation state in the Southern Ocean.

scholarly journals Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean pCO<sub>2</sub>

2017 ◽  
Author(s):  
Amanda R. Fay ◽  
Nicole S. Lovenduski ◽  
Galen A. McKinley ◽  
David R. Munro ◽  
Colm Sweeney ◽  
...  
Keyword(s):  
Time Series ◽  
Southern Ocean ◽  
Seasonal Cycle ◽  
Drake Passage ◽  
Passage Time ◽  
Global Ocean ◽  
Carbon Uptake ◽  
Surface Ocean ◽  
Ocean Carbon

Abstract. The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air-sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize all available pCO2 observations collected in the subpolar Southern Ocean to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long term pCO2 trends shown through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. From 2002–2015, data show that carbon uptake has strengthened with surface ocean pCO2 trends less than the global atmospheric trend in the Drake Passage and the broader subpolar Southern Ocean. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers upstream of the region. We also compare DPT data from 2016 and early 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016–2017, their pCO2 estimates typically fall within the range of underway observations. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment quality control of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink.

scholarly journals Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

2013 ◽  
Vol 10 (9) ◽  
pp. 15033-15076 ◽  
Author(s):  
K. B. Rodgers ◽  
O. Aumont ◽  
S. E. Mikaloff Fletcher ◽  
Y. Plancherel ◽  
L. Bopp ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Southern Ocean ◽  
Ad Hoc ◽  
Global Ocean ◽  
Nutrient Concentrations ◽  
Carbon Uptake ◽  
Carbon Cycle Model ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon ◽  
The Impact

Abstract. Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model. The objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000–2006, with differences of order 0.9 Pg C yr−1 over the region south of 45° S. Wind stirring impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other species associated with ocean biogeochemistry. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong sensitivity of nutrient concentrations exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry, through its impact over the Southern Ocean.

scholarly journals Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean <i>p</i>CO<sub>2</sub>

2018 ◽  
Vol 15 (12) ◽  
pp. 3841-3855 ◽  
Author(s):  
Amanda R. Fay ◽  
Nicole S. Lovenduski ◽  
Galen A. McKinley ◽  
David R. Munro ◽  
Colm Sweeney ◽  
...  
Keyword(s):  
Time Series ◽  
Southern Ocean ◽  
Seasonal Cycle ◽  
Drake Passage ◽  
Passage Time ◽  
Global Ocean ◽  
Carbon Uptake ◽  
Surface Ocean ◽  
Ocean Carbon

Abstract. The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air–sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize available pCO2 observations to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long-term pCO2 trends, as evident through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes both seasonally and annually. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. An increase in winter data would aid in reduction of uncertainty levels. On average over the period 2002–2016, data show that carbon uptake has strengthened with annual surface ocean pCO2 trends in the Drake Passage and the broader subpolar Southern Ocean less than the global atmospheric trend. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers away from the region. We also compare DPT data from 2016 and 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016–2017 compared to the Drake Passage Time-series, their pCO2 estimates fall within the range of underway observations given the uncertainty on the estimates. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment assessment of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink.

scholarly journals Reframing the carbon cycle of the subpolar Southern Ocean

2019 ◽  
Vol 5 (8) ◽  
pp. eaav6410 ◽  
Author(s):  
Graeme A. MacGilchrist ◽  
Alberto C. Naveira Garabato ◽  
Peter J. Brown ◽  
Loïc Jullion ◽  
Sheldon Bacon ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Southern Ocean ◽  
Global Climate ◽  
Dominant Role ◽  
World Ocean ◽  
Future Climate Change ◽  
Carbon Uptake ◽  
Shelf Sea ◽  
Horizontal Circulation ◽  
Ocean Carbon

Global climate is critically sensitive to physical and biogeochemical dynamics in the subpolar Southern Ocean, since it is here that deep, carbon-rich layers of the world ocean outcrop and exchange carbon with the atmosphere. Here, we present evidence that the conventional framework for the subpolar Southern Ocean carbon cycle, which attributes a dominant role to the vertical overturning circulation and shelf-sea processes, fundamentally misrepresents the drivers of regional carbon uptake. Observations in the Weddell Gyre—a key representative region of the subpolar Southern Ocean—show that the rate of carbon uptake is set by an interplay between the Gyre’s horizontal circulation and the remineralization at mid-depths of organic carbon sourced from biological production in the central gyre. These results demonstrate that reframing the carbon cycle of the subpolar Southern Ocean is an essential step to better define its role in past and future climate change.

scholarly journals TREE RING RECONSTRUCTION OF MODERN RADIOCARBON DIOXIDE VARIABILITY OVER THE SOUTHERN OCEAN

2021 ◽  
Author(s):  
Rachel Corran
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Tree Ring ◽  
Deep Water ◽  
Latitudinal Gradient ◽  
Carbon Sink ◽  
Carbon Uptake ◽  
Co2 Uptake ◽  
Anthropogenic Carbon ◽  
Ocean Carbon

<p><b>The Southern Ocean is the largest ocean carbon sink region. However, its trend of increasing carbon uptake has shown variability over recent decades. It is important to understand the underlying mechanisms of anthropogenic carbon uptake such that the future response of the Southern Ocean carbon sink under climate forcing can be predicted. </b></p><p>The carbon uptake of the Southern Ocean is characterised by the balance of outgassing of CO2 from carbon-rich deep water and sequestration of anthropogenic carbon into surface waters. Atmospheric radiocarbon dioxide (Del14CO2) in the Southern Hemisphere is sensitive to the release of CO2 from the upwelling of ‘old’ 14C-depleted carbon-rich deep water at high southern latitudes, but is insensitive to CO2 uptake into the ocean. Thus Del14CO2 has the potential to be used as a tracer of the upwelling observed, thereby isolating the outgassing carbon component. </p><p>The Southern Ocean Region has limited atmospheric Del14CO2 measurements, with sparse long-term sampling sites and few shipboard flask measurements. Therefore in this PhD project I exploit annual growth tree rings, which record the Del14C content of atmospheric CO2, to reconstruct Del14CO2 back in time. Within tree ring sample pretreatment for 14C measurement I automate the organic solvent wash method at the Rafter Radiocarbon Laboratory. I present new annual-resolution reconstructions of atmospheric Del14CO2 from tree rings, from coastal sites in New Zealand and Chile, spanning a latitudinal range of 44 S to 55 S, for the period of interest, 1985 – 2015. Data quality analysis using a range of replicate 14C measurements conducted within this project leads to assignment of apx 1.9 ‰ uncertainties for all results, in line with atmospheric measurements. </p><p>In this project I also develop a harmonised dataset of atmospheric Del14CO2 measurements in the Southern Hemisphere for this period from different research groups, including the new tree ring Del14CO2 records alongside existing data. The harmonised atmospheric Del14CO2 dataset has a wide range of applications, but specifically here allows investigation of temporal and spatial variability of atmospheric Del14CO2 over the Southern Ocean over recent decades, thereby also considering the role of upwelling in recent Southern Ocean carbon sink variability. Backward trajectories are produced for the tree ring sites from an atmospheric transport model, to help inform interpretation of results. </p><p>Over recent decades a latitudinal gradient of 3.7 ‰ is observed between 41 S and 53 S in the New Zealand sector, with a smaller gradient of 1.6 ‰ between 48 S and 55S in the Chile sector. This is consistent with other studies, with the spatial variability of atmospheric Del14CO2 attributed to air-sea 14C disequilibrium associated with carbon outgassing from 14C-depleted carbon-rich deep water upwelling at around 60 S, driving a latitudinal gradient of atmospheric Del14CO2 in the Southern Hemisphere, with longitudinal variability also observed. A stronger atmospheric Del14CO2 latitudinal gradient is observed in the 1980s/1990s relative to later 1990s/2000s. Stronger atmospheric Del14CO2 latitudinal gradients observed in 1980s/1990s suggest stronger deep water upwelling thereby greater associated outgassing of 14C-depleted CO2. These Del14CO2-based observations are consistent with modelling studies that predict changes in deep-water upwelling have controlled decadal variability in CO2 uptake, and are consistent with observation-based studies of decadal changes in rate of CO2 uptake of the Southern Ocean. The results presented in this thesis present the first observation-based confirmation that decadal changes in the strength of deep-water upwelling can explain decadal changes in the rate of CO2 uptake. </p>

scholarly journals Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

2014 ◽  
Vol 11 (15) ◽  
pp. 4077-4098 ◽  
Author(s):  
K. B. Rodgers ◽  
O. Aumont ◽  
S. E. Mikaloff Fletcher ◽  
Y. Plancherel ◽  
L. Bopp ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Southern Ocean ◽  
Ad Hoc ◽  
Global Ocean ◽  
Relative Reduction ◽  
Carbon Uptake ◽  
Carbon Cycle Model ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon ◽  
The Impact

Abstract. Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model, where the objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000–2006, with a relative reduction with wind stirring on the order of 0.9 Pg C yr−1 over the region south of 45° S. This impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other ocean biogeochemical tracers. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong reduction on the order of 25–30% in the concentrations of NO3 exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry through its impact over the Southern Ocean.

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